a. Field of the Invention
The invention is directed toward an apparatus and method for cooling and moving ablation elements in a medical device.
b. Background Art
In a normal heart, contraction and relaxation of the heart muscle (myocardium) takes place in an organized fashion as electrochemical signals pass sequentially through the myocardium from the sinoatrial (SA) node located in the right atrium to the atrial ventricular (AV) node and then along a well defined route which includes the His-Purkinje system into the left and right ventricles. Atrial fibrillation results from disorganized electrical activity in the heart muscle, or myocardium. An increasingly common medical procedure for the treatment of certain types of cardiac arrhythmia and atrial arrhythmia involves the ablation of tissue in the heart to cut off the path for stray or improper electrical signals.
Ablation may be performed either from within the chambers of the heart (endocardial ablation) using endovascular devices (e.g., catheters) introduced through arteries or veins, or from outside the heart (epicardial ablation) using devices introduced into the chest. The ablation devices are used to create elongated transmural lesions—that is, lesions extending through a sufficient thickness of the myocardium to block electrical conduction—which form the boundaries of the conductive corridors in the atrial myocardium. The ablation devices create lesions at particular points in the cardiac tissue by physical contact of the cardiac tissue with an ablation element and the application of energy.
One challenge in obtaining an adequate ablation lesion is the constant movement of the heart, especially when there is an erratic or irregular heart beat. Another difficulty in obtaining an adequate ablation lesion is retaining sufficient and uniform contact with the cardiac tissue across the entire length of the ablation element surface. Without sufficiently continuous and uniform contact, the associated ablation lesions may not be adequate.
An epicardial ablation device may be used to create uniform, continuous, linear lesions during cardiac ablation. The device (e.g., belt) may comprise a plurality of cells connected together by a hinge wire. The hinge wire may comprise nylon or metal and may be provided to connect the cells together so that they are configured to form a substantially complete ring for generally encircling the cardiac tissue at the time of ablation. Each cell may comprise an ablation element, as well as a cell carrier for retaining the ablation element. The device may be positioned securely around a patient's atrium while the ablation elements apply energy (e.g., high intensity focused ultrasound energy) to the targeted tissue. In a conventional epicardial ablation device, the cells typically must be placed as closely together as possible in order to minimize possible ablation gaps between cells. However, the close placement of cells may negatively affect the mechanical flexibility of the device.
In a conventional epicardial ablation device, a membrane is disposed in front of the emitting surface of each ablation element and connected to each cell carrier. Each cell is separately and hermetically sealed with its own designated membrane. Each membrane is generally provided to conform to the required shape to fill a gap between the ablation element and the tissue to be ablated. Each membrane may be fed by an individual fluid inlet leading to the membrane that provides a fluid, such as saline, to the membrane interface. The fluid may flow in the opening between the emitting surface of the ablation element and the membrane in order to provide good acoustic contact, with independent fluid flow in the front and lateral sides of each cell. Multiple fluid inlets are required for the device (i.e., an individual fluid inlet for each cell), which may increase cost and assembly time for the device.
The ablation elements also require cooling in front of the emitting surfaces of the ablation elements. A fluid, such as saline, may serve as a coolant. In addition to flowing in an opening between the emitting surface of each ablation element and the inner surface of each membrane, the fluid may also flow through holes in each membrane (e.g., holes formed by lasers) toward the outer surface of the membrane.